Alternator Wiring?

[30Tudor]

On the old Fordbarn, Steve S posted a great article on alternator wiring tips which I have reproduced below for reference. (Just to let the Model A purists know, I also have a generator, however, I like to experiment and tinker).

I bought a new Nurex 6V alternator last year and recently tried to tie in the exciter circuit that Steve talks about in his article using a diode and a wire from my ignition switch. I definitely have voltage to the exciter terminal at start up, however, the alternator is not “waking up” any better than before. I’m wondering if I have done something wrong or if there is something in the alternator that needs to be changed? I emailed Nurex a couple of weeks ago however, they haven’t responded.

Has anybody else tried this? I would appreciate any help you can provide.

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About wiring an alternator for a Model A—one wire or three?

There have been several posts recently about alternators, so I decided to write up my understanding of what’s involved in hooking up an alternator in the optimum way. This is based only on my own experience and reading. I’m not claiming to be an auto electric expert, and I strongly urge anyone to go over their proposed wiring scheme in detail with a trained professional, like in an auto electric shop, just to be sure. In my experience, those folks are happy to be helpful. I welcome any corrections or clarifications by a real mechanic.

The most popular alternator adapted to old cars is the Delco 10Si, 60 amp, made in 1973-1985, and that is the model I will be talking about. The Delco 10Si has been called the “Model T” of alternators. It is simple, easy, reliable, cheap, widely available, and light weight. Although it is a three-wire (plus ground) design, it can be rigged so that only one wire has to be hooked up, which evidently has immense customer appeal in the after-market, even if it is not ideal. This “single wire” setup works fine for industrial applications or motors that don’t have accessories with remote voltage demands that need to be managed, or motors that idle at 1100 rpm or more at start up. At that high rpm, the residual magnetism in the core of the field coils (the part of the alternator that spins) will be sufficient to allow it to start generating enough current to further magnetize the field coils (self-excite) and start working normally.

However, most Model A’s aren’t set up to idle that fast, so the engine needs to be momentarily goosed to 1200 rpm or so to get the alternator to start working. Even higher rpm are required wake up higher-output alternators. Many think this is a cruel thing to do to a cold engine. Also, the alternator’s voltage regulator will not be working during this period. Some people think that the electrical surge that results from an unregulated alternator suddenly kicking in at high rpm can damage some electrical equipment: I would be particularly concerned about a solid state ignition system. If you share these concerns, it’s not hard to rig the alternator per its original design, so it starts working immediately, even at low rpms.

One, or possibly two, new connections are needed to the internal voltage regulator. The regulator is easily accessed through an opening in the top back of the case. When I got my alternator from Nu-Rex, the opening was sealed with a removable plug. When you pop out the plug, the two lugs you must deal with are seen: the exciter and the sensor, numbered on the case, 1 and 2, respectively.

The sensor’s job is to sense the voltage at the place where it is most important for the voltage to be maintained constant. In modern-era cars, this is the lug of a main junction post from which lights and other accessories derive their power. In Model A usage, the sensor is usually simply connected to the alternator’s BAT pole. That makes a certain amount of sense for the original Model A wiring because the horn and lights are normally also connected there, and it’s good for them to be guaranteed full voltage. On my Nurex alternator, I could see a mystery wire soldered to the sensor lug going somewhere inside the case; it turned out that Nurex had internally wired the sensor to the BAT pole. If your alternator was not purchased ready for one-wire installation, it probably won’t have this internal sensor-to-BAT connection. You can simply connect the sensor by a short loop of wire (12 gauge) outside the case to the BAT pole.

However, before doing that you might want to consider other options. The Model A’s primary ignition circuit is powered from the terminal box on the firewall, which creates a small dilemma: regulate the lighting circuitry or the ignition circuitry? Maybe you don’t care; maybe you have solid state ignition and do care; maybe you have perfect connections and wiring with no voltage losses and it doesn’t matter. If assuring correct voltage to the ignition circuit is your priority, you could run the sensor wire to the terminal box. Or, you could re-route your lighting wires so both the lights and ignition are powered from the same terminal post. But, wherever you connect the sensor, it is important that that is also where the charging wires from the alternator’s BAT post and to the battery are connected. If the sensor goes to the terminal box, and if there is some voltage loss between the alternator’s BAT post and the terminal box, the voltage regulator will increase the BAT post voltage as necessary to keep the terminal box voltage right. If the alternator’s BAT post were then wired directly to the battery (via starter switch), this could result in overcharging the battery.

A slick, so-called DA plug wiring harness is sold by Fifth Avenue Antique Auto Parts (www.fifthaveinternetgarage.com) that helps you wire the sensor, and it also provides a wire with built in diode for making the other new connection that will be needed to the exciter lug. The DA plug also nicely seals the opening. Because of my Nu-Rex’s internal modification, I didn’t need the DA plug’s external sensor wiring. But, also because of that solder connection on the sensor lug, the DA plug didn’t fit, so I couldn’t even use its exciter lug wiring or use the plug to seal the opening. So, I just use a spade lug jack to wire the exciter lug, leave the case opening unsealed, and hope for the best—ok so far, and that includes several leaking radiator events.

The exciter contact must be exposed to at least a few volts at startup to wake up the voltage regulator and start the alternator working without the need to race the engine. Only 2 or 3 volts are needed, but 6 or 12 are ok too; momentary exposure is good enough, but continuous is ok too. The trick is to arrange for the contact to be broken or to otherwise prevent power from draining back from the exciter lug into the primary ignition circuit so the car won’t keep running when you turn off the ignition. There are several ways this can be done. Charlie Yapp (www.secretsofspeed.com) described four in 1997 in his Secrets magazine, vol. 7, #1, with a correction in #2.

The slickest way, and the one I use, is to install a newer, three-position ignition switch that has an accessory ACC position, plus the usual ignition IGN and OFF positions. The General Universal Ignition Switch UN12040 works well, and fits in the Model A dash hole. The exciter is connected to the ACC post. I leave my lights and other accessories wired as before, so only the exciter goes to the ACC. When the key is turned to the IGN position during startup, both the IGN and ACC posts are connected to the battery, so the exciter is seeing the battery voltage. When the engine is turned off, the ACC connection to the battery is broken, and the engine dies like it should. IF the exciter had simply been connected to the IGN post of a regular, two-postion ignition switch, it would work fine for starting the alternator working, but when the engine was turned off, because BOTH the wire to the ignition’s primary circuit and to the exciter are connected on the same IGN post, AND, because the alternator keeps generating current even at low rpm, enough current would feed back from the exciter to power the ignition and keep the engine running.

However, there is a way that you can use your original ignition switch: Install a diode (with the correct polarity) in the line from exciter to IGN so current can only go the right way. Radio Shack # 276-1144 has been suggested. Or, the DA plug wiring harness, with its built-in diode, can be used.

A third approach is to somehow run a wire from the excitor to the contact in the starter switch housing that is only electrically hot when the starter button is being stepped on. During the moments when you are starting the car, the exciter and the starter motor will both see the battery voltage, which will be sufficient to excite the alternator to do its job. This sort of connection would be really easy if the starter used a solenoid. Charlie says a diode is needed here too, but I would have to study a circuit diagram before I understood why; the diode couldn’t hurt, however.

A fourth approach is to connect the exciter to the stoplight switch so that when you step on the brake pedal, juice goes to the exciter as well as the brake lights. The idea here is that you will probably be backing up and using the brakes shortly after starting the car, and that will be soon enough to get the alternator working. Again, a diode or the DA plug harness is needed in the line, in this case to keep the exciter from trying to light the brake lights all the time.

There’s another way that occurs to me that would be ok if you don’t mind one more thing to do in your startup routine. Connect the exciter to any electrically hot post via a “momentary on” push button switch. Then, sometime during startup push the button and the excitor will be momentarily exposed to battery voltage, which should be sufficient to do the job. There must be a bunch more ways to do this, but I think five is plenty.

A discharge indicator light (idiot light) can be installed if you like; unfortunately, I can’t help with the details. My understanding is that it goes in the line from the exciter to the ignition switch, and, depending on the bulb, it might have to be wired in parallel with a resister. Charlie recommends a ten ohm bulb. Basically, the deal is that when the engine is being started, the current flow to the exciter also lights the bulb. After the motor starts and the alternator starts working, both sides of the bulb are at the same voltage so current flow stops and the light goes out. On some setups, if the alternator is overcharging, that voltage imbalance will also light the indicator light, unless the light is an LED, which, like all diodes, only lets current go one way. Sorry I can’t be more help here.

In the picture of my setup, the green wire is the one from the exciter to the ACC post. The brown, orange, and gray ones are ground wires. The BAT terminal on the back of the alternator connects to all of the wires that in the original fastened to the generator cutout: one (yellow on mine) goes up to the terminal box (and then on to the ammeter, starter post, and battery); others (buried in the black plastic loom) go to the horn and the light switch on the end of the steering column. Clearly, I’m well beyond worrying about losing judging points for wiring color correctness! It sure runs good.

http://i29.photobucket.com/albums/c290/sschullery/Alternatoralignment002-closeup.jpg

Finally, a few thoughts for those who have access to or who may be trying to follow Charlie Yapp’s wiring diagrams. I have my alternator wired like his v7.2 “DOES NOT WORK” Diagram 3, except I have the coil between the ammeter and the ignition switch, as in the original. (Charlie’s layout makes more sense to me, but I stuck with the original.) HOWEVER, that scheme did not work for Charlie! The alternator was constantly charging, and the solder inside the main fuse on the alternator side of the fuse holder repeatedly melted. This sounds to me more like a poor connection, or possibly a short, in the fuse holder, than a wiring layout problem, but, for Charlie, the fix was to rewire according to his v7.2, Diagram 1. Rather than running the alternator BAT wire to the firewall junction box, it goes directly to the starter post, bypassing the ammeter and the main fuse. Also, the lights, horn, exciter, and ignition ACC post all are connected at a junction box post. I’m not sure that I fully understand this scheme, but it apparently works for Charlie. For one thing, I don’t like bypassing fuses. For another, it looks to me like the ammeter is in parallel with rather than in series with the alternator-battery-ground electrical path. That means that, other than a momentary twitch, the ammeter will be oblivious to what’s going on between the alternator and battery. For example, I don’t believe that an over-charging situation would register on the ammeter. Also, the headlights and horn are powered through the ignition switch; the General Universal Ignition Switch is rated at 20 amps, which is fairly stout, but I would worry when head lights, tail lights, running lights, turn signals, Pertronix ignition, and other stuff (horn, electric wiper, radio, heater fan?) are all turned on at once.

This may all be moot with the latest generation of rebuilt alternators. I talked to some guy at the Williamsburg MARC meet who was selling alternators that he claimed self-excited at idle speed. As I recall, the claim was that his close machining tolerances allowed the residual magnetism to do the job.

I hope someone finds this useful or interesting.

Steve S
Kalamazoo, MI